Background: One pathway through which pandemic influenza strains might
emerge is reassortment from coinfection of different influenza A viruses. Seasonal
influenza vaccines are designed to target the circulating strains, which intuitively
decreases the prevalence of coinfection and the chance of pandemic emergence
due to reassortment. However, individual-based analyses on 2009 pandemic
influenza show that the previous seasonal vaccination may increase the risk of
pandemic A(H1N1) pdm09 infection. In view of pandemic influenza preparedness, it
is essential to understand the overall effect of seasonal vaccination on pandemic
emergence via reassortment.
Methods and Findings: In a previous study we applied a population dynamics
approach to investigate the effect of infection-induced cross-immunity on reducing
such a pandemic risk. Here the model was extended by incorporating vaccination
for seasonal influenza to assess its potential role on the pandemic emergence via
reassortment and its effect in protecting humans if a pandemic does emerge. The
vaccination is assumed to protect against the target strains but only partially against
other strains. We find that a universal seasonal vaccine that provides full-spectrum
cross-immunity substantially reduces the opportunity of pandemic emergence.
However, our results show that such effectiveness depends on the strength of
infection-induced cross-immunity against any novel reassortant strain. If it is weak,
the vaccine that induces cross-immunity strongly against non-target resident strains but weakly against novel reassortant strains, can further depress the pandemic
emergence; if it is very strong, the same kind of vaccine increases the probability of
pandemic emergence.
Conclusions: Two types of vaccines are available: inactivated and live attenuated,
only live attenuated vaccines can induce heterosubtypic immunity. Current
vaccines are effective in controlling circulating strains; they cannot always help
restrain pandemic emergence because of the uncertainty of the oncoming
reassortant strains, however. This urges the development of universal vaccines for
prevention of pandemic influenza.